1. Field of the Invention
This invention relates to a driving circuit for picture display devices such as Cathode Ray Tubes (CRT) in color television receivers.
2. Description of the Prior Art
In the field of television receivers, receivers have been provided with more and more functions and picture quality has been clearer and clearer. As a new display device, liquid crystal panels are gradually being adapted besides CRT's.
A driving circuit in accordance with the prior art, for use with a picture display device in color television receivers, is explained below.
FIG. 1 is a circuit diagram of a prior art exemplary CRT driving circuit. In FIG. 1, guns for red, green and blue in the CRT are driven by respective driving circuits. As circuit diagrams for the red and blue guns are the same as that for the green gun, only a circuit diagram for the green gun is shown in the drawing.
Transistors Q1 and Q2 are connected in cascade. A capacitor C1 for frequency characteristic compensation and a series connection of a resistor R1 and a potentiometer VR2 are connected between the emitter of the transistor Q1 and the ground. The emitter of the transistor Q1 is connected to the potentiometer VR1 for low light white balance adjustment via a resistor R2. A load resistor R4 is connected between the collector of the transistor Q2 and a power source BH. A video signal at the collector of the transistor Q2 is led to the CRT cathode via a resistor R5.
The action of the CRT driving circuit mentioned above and the white balance adjustment are explained in the following.
As the transistor Q1 and Q2 are connected in cascade, when the resistor R2 is large enough, the gain of the driving circuit is expressed as R4/(R1+VR2), where R4, R1 and VR2 are the resistance values of the resistors R4 and R1 and the potentiometer VR2, respectively. Therefore, the gain is adjustable by the potentiometer VR2.
This driving circuit can be regarded as a d-c amplifier and its d-c offset amount can be varied with the potentiometer VR1. White balance adjustment of this driving circuit is done so that the specified brightness ratios of red, green and blue are obtained at the two brightness levels of low light and high light.
That is, a low light reference signal (flat signal), which level is adjusted at the input of the circuit, is applied to the input of the circuit so that a specified brightness (for example, 5% of the green maximum brightness) is obtained. Then, each potentiometer VR1 for low light adjustment in the red and blue channels is adjusted so that the specified red and blue low light brightnesses are obtained, respectively.
Then, a high light reference signal (flat signal) is applied to the input of the circuit so that a specified brightness (for example, 60% of the green maximum brightness) is obtained by adjusting the potentiometer VR2 for high light adjustment. Then, each potentiometer VR2 in the red and blue circuits is adjusted so that a high light white raster is observed on the CRT screen.
Although adjustment levels for low light and high light were explained as 5% and 60% of the maximum brightness, respectively, in the above example, the adjustment levels are desirably specified so that a good tracking characteristic for white balance is obtained, considering light emitting characteristics of red, green and blue phosphors of a CRT.
However, in the above mentioned circuit, the potentiometer VR2 for high light adjustment varies not only the gain but also the d-c offset amount and the potentiometer VR1 for low light adjustment varies not only the offset amount but also the gain. As high light adjustment and low light adjustment affect each other, a correct adjustment is not obtained unless these adjustments are repeated.